The invention relates to an improved process for preparing a hydroxyl-amine salt. Hydroxyl-amine salts may be prepared by reducing nitrate ions or nitrogen monoxide with hydrogen in the presence of a noble metal catalyst in an acidic medium. The resulting hydroxyl-amine salt may be removed from the reaction mixture. The residual reaction mixture, after removal of the hydroxyl-amine salt, may then be reused.
Hydroxyl-amine salts formed by this reaction may be converted into the corresponding oxime by reacting the hydroxyl-amine salt with cycloalkanone to yield the corresponding oxime. Reactions of this type have been described in the U.S. Pat. Nos. 3,641,150, 3,655,760 and 3,701,809, each of which is hereby incorporated by reference. Hydroxyl-amine salts prepared by the improved method of the present invention are thus useful in the preparation of the corresponding oximes. In turn, these corresponding oximes may be used to prepare nylon 6. Other uses of hydroxyl-amine salts prepared by the improved method of the present invention are reducing agent in photography and antioxidant for fatty acids and soaps.
U.S. Pat. No. 3,767,758 and the publication "Nitrogen" 50, pages 27-30 (1967), suggest that in the preparation of a hydroxyl-amine salt by reduction of nitrate ions or nitrogen monoxide, the selectivity of the reaction is adversely affected by the presence of molybdenum in the reaction mixture. The "selectivity" of the reaction is defined as the yield of hydroxyl-amine salt expressed as percentage of the theoretically possible yield.
The apparatus commonly used in the preparation of hydroxyl-amine salts by reduction of nitrate ions or nitrogen monoxide often is constructed of material containing molybdenum. Corrosion of the apparatus and recirculation of the nitrate/nitrogen monoxide reaction mixture may result in molybdenum contamination of the nitrate/nitrogen monoxide reaction mixture. It has been found that an amount of no more than a few milligrams of molybdenum per liter of nitrate/nitrogen monoxide reaction mixture can adversely affect the selectivity of the reduction reaction. It has been found that the selectivity of the reduction of nitrate ions or nitrogen monoxide can be reduced by the presence of molybdenum contamination by as much as 5 to 15%. The use of apparatus constructed from molybdenum-free materials, to avoid molybdenum contamination of the nitrate/nitrogen monoxide reaction mixture, appears to be very expensive. Heretofore, there has been no economical method of reducing molybdenum contamination in the reduction of nitrate ions or nitrogen monoxide to produce a hydroxyl-amine salt.